Thin-film resistor

ABSTRACT

A thin-film resistor comprising a thin film of a nitride of at least one element belonging to groups III-VI of the periodic table. The thin-film resistor has a metal oxide layer comprising at least one metal oxide selected from the group consisting of manganese oxide, iron oxide, cobalt oxide, nickel oxide, zinc oxide, indium oxide, tin oxide and indium tin oxide interposed between the nitride thin film and an electrode for external connection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin-film resistor, and moreparticularly, it relates to a thin-film resistor provided with a highlyreliable thin-film nitride resistance member whose resistance value isnot substantially changed under high temperature conditions.

2. Description of the Prior Art

A thin film comprising nitrides of elements belonging to groups III-VIof the periodic table such as tantalum nitride, titanium nitride,zirconium nitride, hafnium nitride, aluminum nitride, niobium nitride,boron nitride and chromium nitride is known to be stable under hightemperature conditions and to be excellent in electricalcharacteristics. A highly reliable thin-film resistance member of aprecision type having a small resistance temperature coefficient may beformed from one of these nitrides or from a combination of two or moresuch nitrides. Also, a thin film comprising nitrides of elementsbelonging to groups VII and VIII of the periodic table such as Mn₂ N,Mn₃ N₂, Mn₄ N and Fe₂ N, Fe₄ N, CoN, Co₂ N, Co₃ N₂, Ni₃ N and Ni₃ N₂ isknown to be stable under high temperature and excellent in electriccharacteristics.

Such a thin-film nitride resistance member is formed on an insulatingsubstrate of glass, ceramic material, etc. by a method such as electronbeam deposition, ion beam deposition, flash deposition, cathodesputtering deposition and the like. Such a thin-film resistance membercan also be formed by hot press, sublimate recrystallization, dischargereaction or chemical vapor deposition. In general, such thin-filmresistance members are usually formed through reactive sputteringdeposition performed in an atmosphere of high-purity nitrogen gas andhigh-purity argon.

The thin-film nitride resistance member is provided thereon with anelectrode for external connection, which comprises a multi-layerelectrode of Cr-Cu, Cr-Au, Ni-Cu, Ni-Au, Ni-Ag, NiCr-Au, Ti-Pd-Au,Ti-W-Au and the like. In an external connection electrode having amulti-layer structure, a first layer of Cr, Ni, NiCr or Ti serves as anadhesion layer for the thin-film nitride resistance member and an outerlayer of Cu, Au or Ag serves as a solderable layer.

Such a resistor provided with a thin-film nitride resistance membershows no change in characteristics in lifetime tests such as amoisture-resistance loading test at the room temperature. However, testshave been performed in which the resistance value of such a resistor waschanged when the same was held at a high temperature of, e.g., 150° C.or subjected to a rated voltage loading test at 70° C. Such a phenomenonwas observed in resistors both coated and not coated with insulatingresin and also in a hermetically sealed one, and the resistance valueswere changed at equal rates.

This means that the resistance films were changed under high temperatureconditions. In an effort to find the cause thereof, it has been provedthat the resistance value of such a thin-film nitride resistance memberis changed because nitrogen contained in the resistance film ispartially dissociated in a contact region between the resistance filmand the external connection electrode under high temperature conditions,the nitrogen being transferred to the metal forming the electrode. When,for example, a resistor comprising a thin-film nitride resistance memberof zirconium nitride (ZrN) and an external connection electrode formedwith a first layer of NiCr and a second layer of Au is held at atemperature of 150° C., the color tone of the zirconium nitride thinfilm is changed with time in the vicinity of the external connectionelectrode, from brown to colorless transparency. Such a phenomenon hasbeen analyzed by means such as ESCA and EMX, and it has been found thatnitrogen contained in the zirconium nitride thin film is graduallydissociated and transferred to the NiCr in the external connectionelectrode, causing the color change of the resistance film as well as achange in resistance value.

In other words, the following reaction is caused in the contact portionbetween the thin-film resistance member and the metal of the externalconnection electrode:

    Me.sup.I N+Me.sup.II →Me.sup.I N.sub.I-X +Me.sup.II N.sub.X

(Me^(I) N: thin-film nitride resistance member; Me^(II) : externalconnection electrode)

This is because the external connection electrode is made of metal,which traps nitrogen contained in the thin-film nitride resistancemember upon application of a high temperature so as to nitrogenize theelectrode.

SUMMARY OF THE INVENTION

The inventors have made a study with the object of preventing such aphenomenon, and have found that the aforementioned reaction can beprevented by interposing an intermediate layer such as a stable metaloxide layer, between the thin-film nitride resistance member and theexternal connection electrode.

Accordingly, it is an object of the present invention to provide aresistor having a resistance film comprising a thin-film nitrideresistance member which has small resistance change at a hightemperature.

The present invention is directed to a thin-film resistor comprising athin-film nitride resistance member, an electrode for externalconnection and a conductive metal oxide layer interposed therebetweenand serving as an intermediate layer.

The intermediate layer may be prepared from at least one metal oxideselected from the group consisting of manganese oxide, iron oxide,cobalt oxide, nickel oxide, zinc oxide, indium oxide, tin oxide andindium tin oxide.

In the case of using zinc oxide as the selected one of these materials,the same is advantageously mixed with an additive comprising at leastone oxide selected from the group consisting of iron oxide, zirconiumoxide, indium oxide, tin oxide and lead oxide so as to include 0.5 to99.9 percent by mol of the additive oxide or oxides.

The thin-film nitride resistance member serving as a resistance elementcan be prepared from any of the materials as hereinabove described withreference to the prior art, while the conductive metal oxide layerserving as an intermediate layer must be prepared from a stable metaloxide lower in specific resistance than the thin-film nitride resistancemember.

When, for example, the thin-film nitride resistance member is made ofzirconium nitride, tin oxide may be selected to form the intermediatelayer. When the thin-film nitride resistance member is prepared fromtantalum nitride, indium tin oxide may be selected to form theintermediate layer.

The intermediate layer is generally formed by sputtering, and a targetmaterial selected from various metals or metal oxides described above isemployed to form the intermediate layer from the aforementioned variousmetal oxides. In any case, sputtering may be performed in an atmospherecontaining oxygen. It order to form an intermediate layer of tin oxide,organic tin may be applied by means such as spraying or coating, andthermally decomposed by heat, thereby providing tin oxide.

In addition to the aforementioned sputtering, the intermediate layer maybe formed by dry-type thin film forming means such as vacuum depositionand ion plating.

According to the present invention, a conductive metal oxide layer isinterposed between a thin-film nitride resistance member and anelectrode for external connection, thereby obtaining a stable thin-filmresistor with small deterioration of its characteristics, and morespecifically small resistance deterioration at a high temperature.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows schematically a thin-film resistor according to anembodiment of the invention as further described hereinbelow.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the FIGURE, there is seen a thin-film resistor according toan embodiment of the invention, as further described hereinbelow. Asubstrate 11 has a thin-film resistance member 12 formed thereon. A pairof external connection electrodes 13, 14 are formed at opposite ends ofthe resistance member 12. A pair of intermediate layers 15, 16 areinterposed between the resistance member 12 and the electrodes 13, 14,respectively.

EXAMPLE 1

A thin-film resistance member of zirconium nitride was formed on analumina substrate by performing reactive sputtering with a target ofmetal zirconium in a mixed gas atmosphere of nitrogen and argon underthe following conditions:

substrate temperature: 300° C.

mixed gas ratio: nitrogen/argon=20/80 (volume %)

introduced gas pressure: 1 Kg/cm²

introduced gas flow rate: 20 cc/min.

DC output: 400 W (3.0 W/cm²)

gas pressure: 7.5×10⁻⁴ to 2.0×10⁻² Torr.

Then a mask was placed on the alumina substrate so as to expose aportion where an intermediate layer was to be formed on the thin-filmresistance member of zirconium nitride. Reactive sputtering wasperformed under the following conditions with a target of tin oxide toform an intermediate layer of tin oxide:

substrate temperature: 250° C.

mixed gas ratio: oxygen/argon=40/60 (volume %)

introduced gas pressure: 1 Kg/cm²

introduced gas flow rate: 100 cc/min.

DC output: 500 W (4.0 W/cm²)

gas pressure: 5×10⁻³ Torr.

A metal layer for soldering was formed of Cu on the tin oxide layer asan external connection electrode by vacuum deposition.

A lead wire was soldered to the Cu layer of the thin-film resistor thusobtained, which was then entirely coated with epoxy resin. In thisstate, the thin-film resistor was held at a temperature of 150° C. for1000 hours and then a resistance value was measured in order to compareany change in its resistance value with the measured initial value, withthe result that the rate of change was found to be less than 0.1%.Further, no change was recognized in the color tone of the thin-filmresistor.

EXAMPLE 2

A thin-film resistance member of zirconium nitride was formed on analumina substrate in a manner similar to Example 1.

Then a mask was placed on the alumina substrate to expose a portionwhere an intermediate layer was to be formed on the thin-film resistancemember of zirconium nitride. Reactive sputtering was performed under thefollowing conditions with a target of metal nickel, to form anintermediate layer of nickel oxide:

substrate temperature: 250° C.

mixed gas ratio: oxygen/argon=10/90 (volume %)

introduced gas pressure: 1 Kg/cm²

introduced gas flow rate: 100 cc/min.

DC output: 500 W (4.0 W/cm²)

gas pressure: 5×10⁻³ Torr.

A metal layer for soldering was further formed of Cu on the nickel oxidelayer as an external connection electrode by vacuum deposition.

The thin-film resistor thus obtained was treated similarly to Example 1and held at a temperature of 150° C. for 1000 hours. A resistance valuewas then measured in order to compare any change in its resistance valuewith the measured initial value. The rate of change was found to be lessthan 0.1% similarly to Example 1. Further, no change was recognized inthe color tone of the thin-film resistor.

EXAMPLE 3

Reactive sputtering was performed on alumina substrates under thefollowing conditions with targets of metal tantalum in a mixed gasatmosphere of nitrogen and argon, to form thin-film resistance membersof tantalum nitride having area resistance of 50 Ω/□:

substrate temperature: 300° C.

mixed gas ratio: nitrogen/argon=5/95 (volume %)

introduced gas pressure: 1 Kg/cm²

introduced gas flow rate: 20 cc/min.

DC output: 200 W (2.5 W/cm²)

gas pressure: 0.3×10⁻² to 2×10⁻² Torr.

Then a tin oxide film and a nickel oxide film were formed on theresistance members of tantalum nitride respectively as intermediatelayers, similarly to Examples 1 and 2.

Thereafter metal layers for soldering were formed of Au on therespective intermediate layers as external connection electrodes byvacuum deposition to form two types of thin-film resistors respectively.

Lead wires were soldered to the Au layers of the thin-film resistorsthus obtained. In this state, the thin-film resistors were held at atemperature of 150° C. for 1000 hours to compare any change in theresistance values with the measured initial values. The rates of changewere less than 0.01% respectively.

EXAMPLES 4-17

Thin-film resistance members of various nitrides as shown in thefollowing Table were formed on alumina substrates. Masks were placed onthe alumina substrates to expose portions where intermediate layers wereto be formed on the thin-film nitride resistance members. Thenintermediate layers were formed as shown in the Table. Solderable metallayers as shown in the Table were formed as external connectionelectrodes for soldering lead wires to the metal layers, thereby formingrespective types of thin-film resistors.

                                      TABLE                                       __________________________________________________________________________                              External                                                                            Rate of Change                                     Thin-Film Nitride                                                                         Intermediate                                                                           Connection                                                                          in Resistance                                 Example                                                                            Resistance Member                                                                         Layer    Electrode                                                                           Value                                         __________________________________________________________________________    4    tantalum nitride                                                                          cobalt oxide                                                                           NiCr--Cu                                                                            below 0.01%                                   5    tantalum nitride                                                                          zinc oxide*                                                                            "     "                                             6    tantalum nitride                                                                          indium oxide                                                                           "     "                                             7    tantalum nitride                                                                          manganese oxide                                                                        "     "                                             8    tantalum nitride                                                                          iron oxide                                                                             "     below 0.05%                                   9    titanium nitride                                                                          manganese oxide                                                                        Cr--Cu                                                                              below 0.1%                                    10   titanium nitride                                                                          cobalt oxide                                                                           "     below 0.03%                                   11   titanium nitride                                                                          indium tin oxide                                                                       "     "                                             12   zirconium nitride                                                                         manganese oxide                                                                        Ni--Ag                                                                              below 0.04%                                   13   zirconium nitride                                                                         iron oxide                                                                             "     "                                             14   aluminum nitride                                                                          zinc oxide**                                                                           NiCr--Cu                                                                            below 0.1%                                    15   aluminum nitride                                                                          tin oxide                                                                              "     "                                                               manganese oxide                                             16   zirconium nitride    Al--Au                                                                              below 0.04%                                                    iron oxide                                                                    nickel oxide                                                 17   zirconium nitride                                                                         iron oxide                                                                             "     below 0.05%                                                    cobalt oxide                                                 __________________________________________________________________________     *Zinc oxide contains 5 percent by mol of lead oxide.                          **Zinc oxide contains 1 percent by mol of iron oxide, 1 percent by mol of     zirconium oxide and 2 percent by mol of indium oxide.                    

REFERENCE EXAMPLE 1

A thin-film resistance member of zirconium nitride was formed by themethod described above with respect to Example 1.

Then an NiCr layer was formed on the thin-film resistance member ofzirconium nitride through a mask by vacuum deposition, and a solderableCu layer was formed thereon by vacuum deposition, to form an externalconnection electrode.

A lead wire was soldered to the Cu layer of the thin-film resistor thusobtained, which was then entirely coated with epoxy resin. In whichstate, the thin-film resistor was held at a temperature of 150° C. for250 hours, whereby the thin-film resistor of zirconium nitride waschanged in color from brown to colorless transparency, while itsresistance value was changed over 10% from the measured initial value.

REFERENCE EXAMPLE 2

A thin-film resistance member of tantalum nitride was formed by themethod as described above with reference to Example 3.

Then an NiCr layer was formed on the thin-film resistance member oftantalum nitride through a mask by vacuum deposition, and a solderableAu layer was formed thereon by vacuum deposition, to form an externalconnection electrode.

The thin-film resistor thus obtained was held at a temperature of 150°C. for 1000 hours, whereby the resistance value was changed by 0.5% fromthe initial value.

Although embodiments of the present invention have been described andillustrated in detail, it is clearly understood that the same is by wayof illustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A thin-film resistor comprising:a thin-filmnitride resistance member; an external connection electrode forconnecting said thin-film nitride resistance member with an externalelement; and a metal oxide layer interposed between said thin-filmnitride resistance member and said external connection electrode,wherein said metal oxide layer comprises at least one metal oxideselected from the group consisting of manganese oxide, iron oxide,cobalt oxide, nickel oxide, zinc oxide, indium oxide, tin oxide andindium tin oxide.
 2. A thin-film resistor in accordance with claim 1,wherein said metal oxide comprises a mixture of zinc oxide and about 0.5to 99.9 percent by mol of at least one metal oxide selected from thegroup consisting of iron oxide, zirconium oxide, indium oxide, tin oxideand lead oxide.
 3. A thin-film resistor in accordance with claim 1,wherein said metal oxide layer comprises a stable metal oxide lower inspecific resistance than the thin-film nitride resistance member.
 4. Athin-film resistor in accordance with claim 3, wherein said thin-filmnitride resistance member comprises at least one nitride of an elementselected from the elements in groups III-VI.
 5. A thin-film resistor inaccordance with claim 3, wherein said thin-film nitride resistancemember comprises chromium nitride.
 6. A thin-film resistor in accordancewith claim 3, wherein said thin-film nitride resistance member comprisesat least one nitride of an element selected from the group consisting oftantalum, titanium, zirconium, hafnium, aluminum, niobium, boron, andchromium.
 7. A thin-film resistor comprising:a thin-film nitrideresistance member; an external connection electrode for connecting saidthin-film nitride resistance member with an external element; and anintermediate layer interposed between said thin film nitride resistancemember and said external connection electrode, said intermediate layersubstantially preventing the dissociation of nitrogen from the thin-filmnitride resistance member, and the transfer of such nitrogen to theexternal connection electrode, under high temperature conditions.
 8. Athin-film resistor in accordance with claim 7, said intermediate layerfurther substantially preventing change of the color of the thin-filmnitride resistance member under high temperature conditions.
 9. Athin-film resistor comprising:a thin-film nitride resistance member; anexternal connection electrode for connecting said thin-film nitrideresistance member with an external element; and an intermediate layerinterposed between said thin-film nitride resistance member and saidexternal connection electrode, said intermediate layer limiting a changein the resistance value of the thin-film nitride resistance member underhigh temperature conditions.
 10. A thin-film resistor in accordance withclaim 9, wherein such change in resistance value is limited to less thanabout 0.1 percent.
 11. A thin-film resistor in accordance with claim 10,wherein such change in resistance value is limited to less than about0.05 percent.